Re: [mesa-users] instability issue

[Bill P. <pa...@ki...>]

To follow up on Josiah's excellent response, 

1st) do you really mean 10^-11 seconds?  is that perhaps a typo?
2nd) when I'm doing core collapse supernovae explosions in mesa the timesteps get down to milliseconds at the start.
	the steps grow in size as the timescale of the evolution increases.
	the same is true for other types of evolution -- the timesteps adapt to the timescales of interest according to user set limits.
3rd) there are large differences between mesa's implicit Lagrangian approach and the explicit Eulerian method you seem to have in mind.
	confusion is inevitable until you get clear on this
4th) mesa isn't the right tool for all problems.  you may well decide that your task is one that is better done using something else.

Good luck,
Bill



On Jun 21, 2017, at 8:27 AM, Josiah Schwab wrote:

> Hi Zoe,
> 
>> We are going to study the behaviour of an external field during the
>> evolution of star, therefore a second order equation of time and
>> radius must be solved inside mesa code. The problem is that for the
>> instability to be refused, " dt " must be about 10^(-11) seconds and
>> the timesteps between mesa profiles are of order 10^(+15) seconds. Is
>> it possible to study such a behaviour and apply the equation in mesa?
> 
> It doesn't make much sense to evolve MESA models with a timestep of
> 10^{-11} s.  Perhaps you could use MESA to generate a series of stellar
> models, and then treat everything as "frozen-in" on the short timescale
> of interest for your problem.
> 
>> Another question is about mesa mesh adjustment and the CFL number. How is
>> the CFL number defined in mesa and which quantities does it relate, (dt,dr)
>> or (dt,dm) ?
> 
> If you're using the hydrodynamics capabilities, then there are CFL-like
> controls, e.g.,
> 
>  http://mesa.sourceforge.net/controls_defaults.html#dt_div_min_dr_div_cs_limit
> 
> but otherwise not.  Remember that MESA does not timestep explicitly.
> 
> Best,
> Josiah
> 
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